IQ's whole point is it was supposed to predict how fast kids are gonna learn at school. I would argue testing for people's ability to learn irrelevant stuff in their limited free time is going to rate people's ability to learn which is the very definition of an IQ test (at least what it was meant to be).
Testing people's ability to learn irrelevant stuff in their free time mostly tests how much free time they have and whether or not they're willing to sacrifice it to learn irrelevant stuff.